Abstract
The LEAFY COTYLEDON2 (LEC2) transcription factor with a plant-specific B3 domain plays a central role in zygotic and somatic embryogenesis (SE). LEC2 overexpression induced in planta leads to spontaneous somatic embryo formation, but impairs the embryogenic response of explants cultured in vitro under auxin treatment. The auxin-related functions of LEC2 appear during SE induction, and the aim of the present study was to gain further insights into this phenomenon. To this end, the effect of LEC2 overexpression on the morphogenic responses of Arabidopsis explants cultured in vitro under different auxin treatments was evaluated. The expression profiles of the auxin biosynthesis genes were analysed in embryogenic cultures with respect to LEC2 activity. The results showed that LEC2 overexpression severely modifies the requirement of cultured explants for an exogenous auxin concentration at a level that is effective in SE induction and suggested an increase in the auxin content in 35S::LEC2-GR transgenic explants. The assumption of an LEC2 promoted increase in endogenous auxin in cultured explants was further supported by the expression profiling of the genes involved in auxin biosynthesis. The analysis indicated that YUCCAs and TAA1, working in the IPA-YUC auxin biosynthesis pathway, are associated with SE induction, and that the expression of three YUCCA genes (YUC1, YUC4 and YUC10) is associated with LEC2 activity. The results also suggest that the IAOx-mediated auxin biosynthesis pathway involving ATR1/MYB34 and CYP79B2 does not seem to be involved in SE induction. We conclude that de novo auxin production via the tryptophan-dependent IPA-YUC auxin biosynthesis pathway is implicated in SE induction, and that LEC2 plays a key role in this mechanism.Electronic supplementary materialThe online version of this article (doi:10.1007/s00425-013-1892-2) contains supplementary material, which is available to authorized users.
Highlights
Unlike the spectacular progress that have been made in identifying the key genetic factors that control the reprogramming of somatic cells in animals (Wang et al 2011), the molecular mechanisms involved in the determination of Planta (2013) 238:425–440 plant tissue developmental plasticity are poorly understood
The results showed that LEAFY COTYLEDON2 (LEC2) overexpression severely modifies the requirement of cultured explants for an exogenous auxin concentration at a level that is effective in somatic embryogenesis (SE) induction and suggested an increase in the auxin content in 35S::LEC2-glucocorticoid receptor (GR) transgenic explants
The analysis indicated that YUCCAs and TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS1 (TAA1), working in the IPAYUC auxin biosynthesis pathway, are associated with SE induction, and that the expression of three YUCCA genes (YUC1, YUC4 and YUC10) is associated with LEC2 activity
Summary
Unlike the spectacular progress that have been made in identifying the key genetic factors that control the reprogramming of somatic cells in animals (Wang et al 2011), the molecular mechanisms involved in the determination of Planta (2013) 238:425–440 plant tissue developmental plasticity are poorly understood. LEC2 directly triggers the expression of seed-specific genes (Santos-Mendoza et al 2005) and controls oil and protein metabolism in maturing seeds (Kroj et al 2003; Baud et al 2007). LEC2 is expressed in zygotic embryos between 4 and 14 days after pollination in a developmentally regulated pattern (Stone et al 2001; Kroj et al 2003), and different genetic factors repress its expression in post-embryogenic tissue, including PICKLE (PKL), a chromatin remodeler (Ogas et al 1999) and fertilisation-independent endosperm (FIE), the Arabidopsis homolog of the PRC2 complex (Bouyer et al 2011)
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